Microstructure And Mechanical Properties Of TC18 Near β Titanium Alloy Fabricated By Selective Laser Melting

Aircraft are developing towards the trend of large scale,high speed/high motorization and complex structure,and the requirements for comprehensive mechanical properties of structural materials are increasing.TC18（Ti-5Al-5Mo-5V-1Cr-1Fe）titanium alloy has the advantages of light weight,high strength and high toughness,which is used in landing gear,fuselage butt frame,beam and other structures of aviation aircraft.In recent years,the application of complex structures such as personalization,complexity and abnormity in aircraft is increasingly widespread.However,TC18 titanium alloy complex structural parts still face some problems in the processing process,such as low material utilization rate,long cycle and high manufacturing cost,which have formed certain constraints on its wide application in the aeronautical field.Additive manufacturing technology has the advantages of unrestricted part shape,short design response cycle and high integration,and is becoming an important technology for fabricating complex structural parts of titanium alloy.In this paper,Selective Laser Melting（SLM）additive manufacturing technology was used to fabricate TC18 titanium alloy.The microstructure,mechanical properties and anisotropy of TC18 nearβtitanium alloy fabricated by SLM were studied.The effect of SLM forming process on microstructure and mechanical properties of TC18 titanium alloy was analyzed.The effect of heat treatment on microstructure evolution and mechanical properties of TC18 titanium alloy fabricated by SLM was clarified.The effect of heat treatment on the anisotropy of microstructure and mechanical properties of TC18 titanium alloy was investigated.The main innovation conclusions are as follows:（1）The effects of volumetric energy density and scanning strategy on the microstructure and mechanical properties of SLMed TC18 titanium alloy were revealed.It was found that volumetric energy density should be controlled within a certain range to avoid the formation of defects and grain growth.The density of the printed alloy is above 99.6%.When the volume energy density is too low（≤55.6 J/mm³）,there is unfusion defect,which leads to the decrease of density.When the bulk energy density is too high（≥76.4 J/mm³）,there are some defects such as pores,which lead to the decrease of density,accompanied by grain coarsening.The elongation and hardness of SLMed alloy are proportional to its density.When the volumetric energy density is too high,the tensile strength and elongation decrease with the increase of volumetric energy density.Moreover,the rotation angle between layers affects texture intensity and grain size.When the rotation angle between layers increases from 0°to 67°,the texture intensity decreases,and the average size of columnar grains decreases from112.7μm to 67.6μm.When increasing to 90°,the texture intensity increases,and the average size of columnar grains increases from 67.6μm to 95.8μm.The texture intensity of alloy fabricated by 2×2 mm²checkerboard scanning strategy is higher than that of strip scanning strategy.The SLMed alloy has fullβcolumnar grain with<001>silk texture,and theβcolumnar grain is composed of submicron cellular structure（about 500 nm）.（2）The effect of aging temperature on microstructure evolution and mechanical properties of TC18 titanium alloy was revealed,and it was found that double-stage aging was an effective method to obtain good comprehensive mechanical properties.The aging temperature can promote the transformation ofβphase intoωphase andαphase.When the aging temperature is lower than 450℃,αphase,βphase and isothermalωphase exist in the aged TC18 alloy.When the aging temperature is higher than550℃,the isothermalωphase gradually disappears,and there are mainlyαandβphases in the aged TC18 alloy.Increasing aging temperature can promoteαphase coarsening.When the aging temperature increases from600℃to 750℃,the average length of theαlaths increases from 402 nm to 1860 nm and the average thickness increases from 120 nm to 373 nm.The average width of grain boundaryαincreases from 0.15μm to 0.35μm,and the width of precipitation free zone ofαphase also increases.When the aging temperature is 450～650℃,the tensile strength of the low-temperature aged alloy can reach 1496 MPa,but its elongation is poor and does not exceed 4.3%.After double-stage aging at 750℃/600℃,the alloy precipitates micrometer-scaleαlaths and nano-scaleαlaths.Nano-scaleαlaths hinder dislocation slip and improve the tensile strength,whileβmatrix and micrometer-scaleαlaths can improve elongation,resulting in good comprehensive mechanical properties.The tensile strength is about1264 MPa,and the elongation is up to 9.3%.（3）The effect of heat treatment on the anisotropy of microstructure and mechanical properties of TC18 titanium alloy was revealed,and the corresponding evaluation method for the anisotropy of mechanical properties was established.The SLMed alloy exhibits slight anisotropy in mechanical properties.The results of X-ray diffraction analysis show that the total dislocation density of sample with vertical orientation(3.1×10¹⁵m^-2)is higher than that of the sample with horizontal orientation(2.2×10¹⁵m^-2),so that the yield strength of vertical sample（868±7 MPa）is higher than that of horizontal sample（815±24 MPa）.However,compared with the horizontal samples,the vertical samples showed similar tensile strength and lower elongation.After aging treatment,the precipitation ofαphase weakens the texture intensity of TC18 titanium alloy,the total dislocation density is similar,and the mechanical properties tend to be isotropic.A method was established to evaluate the anisotropy of mechanical properties of TC18 titanium alloy fabricated by SLM,and the degree of yield strength anisotropy was verified by the maximum and minimum values of total dislocation density in different forming directions.There were 100 figures,16 tables and 264 references...